Articulated hold-down anchor device for the embedded cables of a prestressed concrete girder

ABSTRACT

An articulated hold-down anchor device for the embedded cables of a prestressed concrete girder, wherein the frame which supports the usual hold-down rollers for the various tiers of tensioned cables is of a special free floating nature so that it readily bisects the direction-change angle which it creates in the cables as they are undergoing tensioning. In this manner, lateral stresses on the frame are equalized so that no bending forces are applied to the anchor bolt which serves to connect the device to the concrete form in which the girder is produced by pouring wet concrete into the form and around the prestressed cables.

United States Patent 1 1 1111 3,831,331

Colado Aug. 27, 1974 ARTICULATED HOLD-DOWN ANCHOR 3.286.419 11/1966 Eriksson 52/225 DEVICE FOR THE EMBEDDED CABLES OF A PRESTRESSED CONCRETE GIRDER Joseph J. Colado, Addison, Ill.

Superior Concrete Accessories, Inc., Franklin Park, Ill.

Filed: Jan. 29, 1973 Appl. No; 327,813

Inventor:

Assignee:

US. Cl 52/225, 254/190, 24/115 E Int. Cl. E04c 5/08, E04c 5/ 16 Field of Search 52/225, 226, 699;

References Cited UNITED STATES PATENTS ll/l9l3 Smith 254/190R l0/l96l Gerwick 10/ l 96! Hillberg Hillberg 52/225 Primary ExaminerFrank L. Abbott Assistant ExaminerJames L. Ridgill, Jr. Attorney, Agent, or Firm-Norman H. Gerlach ABSTRACT An articulated hold-down anchor device for the embedded cables of a prestressed concrete girder, wherein the frame which supports the usual holddown rollers for the various tiers of tensioned cables is of a special free floating nature so that it readily bisects the direction-change angle which it creates in the cables as they are undergoing tensioning. In this manner, lateral stresses on the frame are equalized so that no bending forces are applied to the anchor bolt which serves to connect the device to the concrete form in which the girder is produced by pouring wet concrete into the form and around the prestressed cables 1 Claim, 4 Drawing Figures I ARTICULATEDHOLD-DOWN ANCHOR DEVICE FOR THE EMBEDDED CABLES OF A PRESTRESSED CONCRETE GIRDER The present invention relates to a hold-down anchor device by means of which the tensioning cables in a prestressed concrete girder are held taut and in their proper relative positions within the girder form during initial pouring and subsequent hardening of the girderforming concrete so that when the tension in the cables is relieved after the concrete has become set and the projecting ends of the cable have been severed from the girder ends, the concrete of the girder will remain in a so-called prestressed condition, thus providing a finished concrete girder which possesses materially greater strength than a conventional girder which is not prestressed in this manner.

In the construction of a prestressed concrete girder of the type set forth above, the tensioning cables are ordinarily strung longitudinally through the girder form in different but generally longitudinally extending directions. For example, in a prestressed concrete beam-type girder having an upper head portion, a lower base portion, and a narrow interconnectingweb portion between the two portions, certain of the cables are strung through the girder form so that they extend inwardly from the opposite ends of the girder in the head portion thereof and then are inclined downwardly to the end that their central or intermediate portions pass through the narrow web portion of the girder and enter the base portion, after which they extend longitudinally in straight line fashion through the base portion of the girder in the medial region of the latter. The cables are thus caused to make a dip, so to speak, as they pass through the girder from one end to the other so that the end regions of the cables are embedded in the girder head portion while the medial regions are embedded for the most part in the girder base portion.

In order to impart to the cables the aforementioned directional characteristics, it is customary to utilize at least two hold-down anchor devices which are bolted to and extends upwards from the bottom or base member of the concrete form and are in the form of rigid, vertically elongated frames embodying vertically spaced hold-down rollers beneath which the tier of tensioning cables extend. These frames extend substantially vertically and in most instances they-are in fixed relation with respect to the base member of the form and the various rollers which are carried thereby impart a direction change to the cables as the latter are progressively tensioned. Since the frames are as a general rule fixed, it is obvious that unless the angle of incidence of the cables equals the angle of emergence, a lateral thrust will be applied to the fixed frames, the extent of such thrust being appreciable when it is considered that the upward thrust on each roller frequently amounts to as much as 3,000 pounds- Such a magnitude of consequent lateral thrust on the frames of the two hold-down anchor devices frequently results in bending of the'anchor bolts which hold the frames in position on the base member of the concrete form.

According to the present invention, special means are provided whereby the roller-supporting frame of each hold-down anchor device is permitted to yield to the unequal lateral thrust which is applied thereto and is incident to direction changes in the associated cables with the result that at such time as the cables are fully member and the provision of a diametric transverse catensioned, the frame will bisect tha deviation angles of the cables. in other words, the cable angle of incidence will equal the angle of emergence whereby no unequal stresses will be imparted to the frame and the resultant bending stress which is applied to the anchor bolt will be a minor one, the pull on the bolt being largely an upward one.

in an instance where the frame of the hold-down anchor device is fixed, it is obvious that the extent of lateral force which is applied to the frame will increase in magnitude as the distance of the rollers from the proximate or lower end of the frame increases. Thus the uppermost roller will exert the greatest bending force on the anchor bolt. With the present hold-down anchor device, since the frame is a free floating frame as indicated above, the frame in its entirety will seek an ultimate position of slight inclination where all resultant cable forces are precisely longitudinal in their direction and the slight departure of the frame from its normal vertical position is not of such an extent that any appreciable degree of bending stress is applied to the bolt for anchoring the device to the base member of the concrete form.

Still further, according to the present invention, novel anchor means are provided for holding the rollercarrying frame in a freely floating condition, such anchor means embodying in addition to the regular anchor bolt, (1) an elongated, tubular, internally threaded, spacer column the lower end of which is adapted to receive the shank of the associated anchor bolt and the upper end of which embodies a transverse bore, and (2) a sturdy through-bolt which extends through said bore and aligned holes in the lower ends of the side members of the frame and forms a pivotal connection between the lower end of the frame and the upper end of the column whereby the frame as a whole may tilt freely with respect to the column and associated anchor bolt.

Due to the fact that, as previously pointed out, lateral stresses in the lower region of any hold-down anchor device of the character under consideration are small, the spacer column itself is employed as a hold-down ble-receiving bore in the upper end of the spacer column, affords a convenient hold-down shoulder for the lowermost cable in the group of cables, the lateral stress which arises incident to tensioning of such cable being insufficient to apply any appreciable degree of bending force to the spacer column or to the anchor bolt by means of which such spacer column is anchored in position on the base member of the concrete girder form.

The provision of a hold-down anchor device such as has briefly been outlined above, and possessing the stated advantages, constitutes the principal object of the present invention.

Other objects and advantages of the invention, not at this time enumerated, will readily suggest themselves as the nature of the invention is better understood from a consideration of the following detailed description.

FIG. 1 is a fragmentary sectional view taken substantially centrally and longitudinally through a concrete girder form, showing a pair of hold-down anchor devices embodying the present invention operatively installed therein;

FIG. 2 is an enlarged fragmentary side elevational view of one of the improved hold-down anchor devices, such view particularly showing the lower base region of the device;

FIG. 3 is a transverse sectional view taken on the line 33 of FIG. 2; and

FIG. 4 is a transverse sectional view taken on the line 4-4 of FIG. 2.

Referring now to the drawings in detail and in particular to FIG. 1, a prestressed concrete girder utilizing a number of hold-down anchor devices embodying the present invention is fragmentarily illustrated and is designated in its entirety by the reference numeral 10. The girder is shown as being in position within a form which is initially prepared to receive the concrete of the girder during pouring operations. Only the flat bottom or base member 12 of the form is shown in FIG. 1 and only two of the hold-down anchor devices appear in this view. For purposes of description herein, it may be assumed that the girder is in the form of an I-beam and that the section represented by this view is taken longitudinally and centrally through the web and lower base portions of the girder. For a full disclosure of a prestressed concrete girder having the configuration of an I-beam and utilizing hold-down anchor devices in the web and lower base portion thereof, reference may be had to US. Pat. No. 3,006,014 granted on Oct. 31, 1961, and entitled HOLD-DOWN ANCHOR DE- VICE FOR THE EMBEDDED CABLES OF PRE- STRESSED CONCRETE GIRDERS."

As disclosed in FIG. 1 of the accompanying drawings, tensioning cables 14 in the form of a group extend in a generally longitudinal direction completely through the girder 10 from end to end and assume various directional positions within the girder as will be described presently. The cables are preferably comprised of stranded steel wire. They are firmly embedded in the surrounding mass of concrete and are initially positioned in the concrete girder form by means of the aforementioned hold-down anchor devices. each of which is designated in its entirety by the reference numeral 16. These anchor devices 16 are in the form of composite anchor assemblies, each assembly being designed to accommodate a plurality of the tensioning cables 14. The illustrated cables 14 extend from the upper regions of the girder at one end thereof inwardly and downwardly on an incline through the web portion of the l-beam girder and into the base portion of the girder where they are engaged by one of the hold-down devices 16, after which these cables extend horizontally along the girder base portion to the other illustrated anchor device. From the second device, the cables extend upwardly and outwardly and into the head portion of the girder where they emerge from the girder.

Each hold-down anchor device 16 is in the form of a frame-like structure 18 including a pair of vertically elongated side plates 20 and 22, each plate comprising a relatively long narrowstrip of heavy gauge sheet or bar steel. The side plates 20 and 22 of the frame-like structure 18 are maintained in their spaced apart parallel relationship by means of a plurality of equally and vertically spaced roller-supporting spacer and holddown assemblies each of which includes a series of three contiguous spacer sleeves, a nut and bolt assembly, and a hold-down roller. As best shown in FIG. 3, each such assembly comprises a horizontal bolt having a shank portion 30 which projects through a pair of aligned holes 32 in the side plates 20 and 22, a bolt head 35 at one end of the shank, and a nut 37 at the other end of the shank. A central spacer sleeve 34 of small outside diameter is telescopically received on the central portion of the bolt shank 30 and is flanked by a pair of spacer sleeves 36 of larger outside diameter, the nut 36 and the bolt head 34 serving to draw the side plates 20 and 22 hard against these sleeves 34 and 36. A hold-down roller 38 with a grooved periphery is rotatably mounted on the central spacer sleeve 34 and is of slightly less thickness than the longitudinal extent of such sleeve so that it may have freedom of rotary motion. As shown in FIG. 1, the various cables 14 pass beneath the rollers 38 and, when they are tensioned, they exert thereon an upward thrust of great magnitude.

As shown in FIGS. 2 and 4 of the drawings, the frame-like structure 18 is adapted to be affixed to the base member 12 of the girder form by means of an upstanding, tubular, supporting column 40 which is of cylindrical design and embodies a threaded axial bore 42 therethrough. The column 40 is adapted to stand on end on the base form member 12 and to be clamped in position by means of a vertically extending anchor bolt 44 which passes upwardly through an opening or hole 46 in said base member and is threadedly received in the axial bore 42 of the column 40. A horizontally extending through-bolt assembly serves pivotally to connect the tubular column 40 to the frame-like structure 18, such assembly embodying a bolt having a bolt head 48, a shank 50 and a nut 52. The shank 50 passes through aligned holes 54 in the lower end regions of the side plates 20 and 22 and receives thereover the upper end region of the column 40, the latter being formed with a transverse bore 56 which intersects the bore 42 and receives therethrough the bolt shank 50. The outside diameter of the tubular column 40 is slightly less than the distance between the two side plates 20 and 22 so that the column has limited freedom of sidewise shifting movement on the bolt shank 50 and will not bind against said side plates. A second transverse bore 60 (see FIGS. 1 and 2) extends through the tubular column 40 a slight distance below the transverse bore 56, extends at a right angle to such bore 60 and is adapted to receive therethrough the lowermost cable 14 in the series of cables.

In the installation and use of the hold-down anchor devices 16, holes such as those shown at 46 are drilled or otherwise formed in the bottom or base member 12 of the concrete girder form in the regions where it is desired to install the devices 16. The bolts 44 are passed upwardly through the holes and are threadedly received in the threaded bores 42 of the columns 40. When the bolts are tightened, the lower ends of the columns rest squarely and firmly on the upper surface of the base member 12 so that the columns extend vertically and are rigidly secured within the empty concrete form between the form side walls (not shown). Thereafter, the various cables 14 are threaded through the frame-like structures 18 so that each cable underlies an associated hold-down roller 38 with the exception of the lowermost cable which is threaded through the transverse bores 60 in the columns 40. Upon tensioning of the cables 14 by means of the usual hydraulic tensioning mechanism (not shown), the cables will be drawn upwardly against their associated rollers 38 and, as the tension is progressively applied, the rollers will turn on the central spacer sleeves 34 and give rolling fractional support to the cables. Because of the fact that each frame-like structure 18 is pivotally connected to the associated column 40 the structure 18 will seek a position of equilibrium wherein the upward thrust of the cables upon the rollers 38 will cause the frame-like structure 18 to bisect the deviation angle which is established in each cable at its regions of contact with the associated rollers. Stated otherwise, the resultant angle of incidence of each cable will equal the angle of emergence so that the upward pull on the frame-like structures 18 as a whole will be precisely longitudinal.

Ordinarily the inclination of the frame-like structures 18 of the devices 16 will be extremely small and, there fore, no appreciable bending stresses are applied to the tubular columns 40 or the bolts 44 by means of which they are held in position on the base member 12 of the girder form.

It is obvious that where a rigid vertical frame-like structure is employed for supporting a series of holddown rollers, the lateral thrust which is applied to such structures is a direct function of the distance which any given roller assumes above the lower end of the structure. The uppermost roller, for example, will exert an appreciable lateral thrust on the frame-like structure 18 since it yields a large moment arm, whereas the lowermost roller which yields only a small moment arm will not exert an appreciable lateral thrust on the structure. Therefore, in the present instance where the lowermost cable 14 is threaded slidingly through the transverse bore 60 of the fixed column 40, only a very small degree of lateral thrust is applied to such column.

The invention is not to be limited to the exact arrangement of parts shown in the accompanying drawings or described in this specification as various changes in the details of construction may be resorted to without departing from the spirit or scope of the invention. Therefore, only insofar as the invention is particularly pointed out or defined in the accompanying claim is the same to be limited.

Having thus described the invention what I claim as new and desire to secure by Letters Patent is:

1. A hold-down anchor device adapted for positioning the superimposed tensioning cables of a prestressed concrete girder within a concrete girder form preparatory to concrete-pouring operations, and comprising a tubular supporting column having formed therein a threaded axial bore adapted to receive the threaded end of a fastening bolt whereby the column may be fixedly secured to the base member of the girder form in upstanding vertical relationship within the confines of the form, and an elongated frame-like structure pivotally connected at its lower end to the upper end region of said column and comprising a pair of spaced apart, parallel, side plates the lower'end portions of which straddle the upper end region of said column, said upper end region of the column being formed with a transverse bore therethrough, a pivot bolt projecting through said lower end portions of the side plates and the transverse bore and constituting pivotal means whereby the frame-like structure as a whole is pivotally connected to the column, said side plates being provided with vertically spaced pairs of transversely aligned holes therein, a transverse clamping bolt having its shank portion projecting through each pair of transversely aligned holes, a central spacer sleeve mounted on the shank portion of each bolt midway between said side plates, a hold-down roller rotatably mounted on each spacer sleeve, and a pair of additional spacer sleeves on the shank portion of each clamping bolt, flanking the central spacer sleeve on opposite sides thereof, said additional spacer sleeves being of larger external diameter than that of the adjacent central spacer sleeve and thus establishing annular shoulders which maintain such roller on the central spacer sleeve, the upper end region of the supporting column being provided with a second transverse bore which extends in a direction at a right angle to the direction of the first transverse bore, said second transverse bore being designed for reception of the lowermost cable in the superimposed series thereof, each of said rollers being adapted to overlie and tractionally engage one of the tensioni'ng cables so as to receive the upward thrust thereof and space the same from said base member when the associated cable is drawn taut, whereby the frame-like structure will be constrained to swing about the axis of said pivot bolt and assume a position wherein the angle of cable incidence is equal to the angle of cable emergence from said frame-like support. 

1. A hold-down anchor device adapted for positioning the superimposed tensioning cables of a prestressed concrete girder within a concrete girder form preparatory to concrete-pouring operations, and comprising a tubular supporting column having formed therein a threaded axial bore adapted to receive the threaded end of a fastening bolt whereby the column may be fixedly secured to the base member of the girder form in upstanding vertical relationship within the confines of the form, and an elongated frame-like structure pivotally connected at its lower end to the upper end region of said column and comprising a pair of spaced apart, parallel, side plates the lower end portions of which straddle the upper end region of said column, said upper end region of the column being formed with a transverse bore therethrough, a pivot bolt projecting through said lower end portions of the side plates and the transverse bore and constituting pivotal means whereby the frame-like structure as a whole is pivotally connected to the column, said side plates being provided with vertically spaced pairs of transversely aligned holes therein, a transverse clamping bolt having its shank portion projecting through each pair of transversely aligned holes, a central spacer sleeve mounted on the shank portion of each bolt midway between said side plates, a hold-down roller rotatably mounted on each spacer sleeve, and a pair of additional spacer sleeves on the shank portion of each clamping bolt, flanking the central spacer sleeve on opposite sides thereof, said additional spacer sleeves being of larger external diameter than that of the adjacent central spacer sleeve and thus establishing annular shoulders which maintain such roller on the central spacer sleeve, the upper end region of the supporting column being provided with a second transverse bore which extends in a direction at a right angle to the direction of the first transverse bore, said second transverse bore being designed for reception of the lowermost cable in the superimposed series thereof, each of said rollers being adapted to overlie and tractionally engage one of the tensioning cables so as to receive the upward thrust thereof and space the same from said base member when the associated cable is drawn taut, whereby the frame-like structure will be constrained to swing about the axis of said pivot bolt and assume a position wherein the angle of cable incidence is equal to the angle of cable emergence from said frame-lIke support. 